Method of producing refractory bodies having controlled porosity



3,535,110 METHOD OF PRODUCING REFRACTORY BODIES HAVING CONTROLLEDPOROSITY Hoyt H. Todd, La Habra, Calif., assignor, by mesne assignments,to the United States of America as represented by the Administrator ofthe National Aeronautics and Space Administration No Drawing. Filed June9, 1967, Ser. No. 644,799

Int. Cl. C22c 29/00 US. Cl. 75-202 22 Claims ABSTRACT OF THE DISCLOSUREA method for producing a porous body of selected pore size andconcentration and large surface area and the body produced thereby whereinitially a porous body is formed by pressing a mixture of finerefractory metal powder such as tungsten and fine inert metal powdersuch as copper or fine boron composition powder such as boron nitride orboth and then sintering the resulting compacted body.

BACKGROUND OF THE INVENTION Origin of the invention The inventiondescribed herein was made in the performance of work under a NASAcontract and is subject to the provisions of Section 305 of the NationalAeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42U.S.C. 2457).

Field of the invention The field of the present invention is powdermetallurgy methods and the bodies produced by such methods. The uses ofsuch bodies are high temperature structures such as electrodes, lampfilaments, rocket nozzles, turbine blades, and bearings.

Description of the prior art It has been recognized for many years thatchemical rocket engines are not suitable for long range space missionsbecause of the enormous amounts of fuel they require. Consequently,efforts have been directed toward the development of electric rocketengines which are capable of generating thrust over long periods of timewith a small expenditure of fuel. The presently known most efficientelectric rocket engine is the ion engine wherein fuels such as cesiumare converted from neutral atoms to ions and then such ions areaccelerated through an electric field to provide the thrust. Onewell-known way of forming an ion from a neutral atom, i.e., stripping anelectron from the atom, involves the surface ionization process whereinthe atom is brought in to contact with a suitable heated ionizer surfacesuch as tungsten and such contact causes the electron to leave the atom.The presently preferred method of contacting the atom with the ionizersurface involves diffusing a cesium vapor through a heated, finelyporous plug of tungsten with the result that the cesium ions aredischarged from the tungsten surface into an adjoining electric field. Abrief description of the operation of such ion engine is to be found inInternational Science and Technology, January 1964, pp. 52-59.

From the foregoing brief discussion, it can be seen that for the properoperation of an ion engine, it is necessary to have a heated finelyporous plug of material such as tungsten through which a vapor such ascesium vapor may diffuse and evaporate from the frontal surface as ions.After considerable experience, it has been found that a very smallaverage pore size and a 3,535,110 Patented Get. 20, 1970 very highaverage pore concentration are necessary to obtain the desired degree ofionization when the vapor passes through the porous plug. Thus, forexample, it has been found that for ionizing cesium vapor by passing itthrough a tungsten plug, an average pore size of about 1 to 5 micronswidth with a ratio of pore surface area to solid surface area in therange of about 0.50 to 0.75. Such selected pore size and concentrationas well as large surface area may be achieved by the straight forwardpowder metallurgy technique of pressing and sintering a fine tungstenpowder in the size range of about 1 to 5 microns; however, when suchtechnique uses the conventional hydrogen-reduced or angular finetungsten powder, the resulting porous body is unstable at hightemperatures. Such instability is due to the high free surface energypresent because of the large surface area of such fine powders andresults in the densification of the porous body so that over a period oftime both the porosity and permeability are substantially decreased. Asubstantial improvement in the stability problem was first achieved bythe use of spherical tungsten powder rather than angular tungsten powdersince it has substantially less surface area for the same size range ofparticles. However, despite such improvement, the attainment of stableporous bodies with such selected pore size and concentration continuedto present substantial problems. One problem was that even usingspherical tungsten powder, the porous body was unstable for long periodsof operation at high temperature. A second problem, particularly withrespect to the commercial utilization of such porous bodies was thatsuch fine spherical tungsten powder is very expensive as compared to theusual angular tungsten powder. For example, spherical tungsten powder ofless than about 1 micron diameter costs on the order of $1,000 per poundwhile comparable angular tungsten powder costs about $4 per pound.Consequently, extensive research was conducted along the line of theaddition of an inhibiting material which would retard the densificationof the porous body without altering its other surface characteristics.Thus, the addition of minor amounts of metals such as irridium, rhenium,chromium, aluminum, and tantalum, and combinations thereof, were testedwith limited success. For example, the addition of small amounts offinely powdered tantalum and rhenium to the fine spherical tungstenpowder substantially reduced the rate of densification at hightemperatures but at the expense of a greatly altered microstructure ofthe porous body.

SUMMARY OF THE INVENTION Consequently, an object of the presentinvention is an inexpensive, stable, porous body having a selected poresize and concentration and large surface area.

A more specific object of the present invention is a porous body havinghigh porosity and permeability with substantially all of its pores beingelongated but their length to width ratio is greater than about 5 and/orsubstantially all of its surface area coated by a boron composition.

Still another object of the present invention is a method for producingan inexpensive, stable, porous body having a selected pore sizeconcentration and large surface area.

Thus, in general, the present invention involves the method forproducing a porous body having a selected pore size and concentrationand a large surface area and the porous body produced by such methodwhich includes forming a porous body comprising a mixture of finerefractory metal powder and either fine boron composition powder or fineinert metal powder, or both. Said inert metal is substantially insolublein the refractory metal and has a melting temperature above thesintering temperature of said refractory metal. The formed body is thenheated to a temperature above such sintering temperature and below suchmelting temperature for a time period adapted to establish a sinteringpattern in the body. Finally, the body is heated to a temperature abovethe melting temperature for a time period adapted to substantiallycompletely evaporate the inert metal powder and sinter the refractorymetal powder to a selected pore size.

DESCRIPTION OF THE PREFERRED EMBODIMENTS One method of the presentinvention involves first forming a porous body comprising a mixture offine refractory metal powder and fine inert metal powder. Such formingmay be done by the usual powder metallurgy technique of pressing theporous body to the desired configuration at a pressure in the range of40,000 p.s.i. to 80,000 p.s.i. The refractory metal used may be selectedfrom the class consisting of Groups IVB, VB, VIB, VIIB, and VII metals.The Group IV-B metals include titanium, zirconium, and hafnium. TheGroup V-B metals include vanadium, columbium, and tantalum. The GroupVIB metals include chromium, molybdenum, and tungsten. The Group VII-Bmetals include manganese, technetium, and rhenium. The Group VIII metalsinclude iron, cobalt, nickel, ruthenium, rhodium, palladium, oxmium,iridium, and platinum. The preferred refractory metals are tungsten andmolybdenum. The particle size of the fine refractory metal powderdepends upon the selected pore size and concentration. In the case ofthe tungsten ionizer, for the ion engine described above, angulartungsten powder of about 1 micron or less diameter is used. Sphericaltungsten powder of the same size, may, of course, also be used; however,its high cost practically prohibits its use because of the improvedresults obtained by the present invention. The inert metal used in saidmixture must be substantially insoluble in the refractory metal and havea melting temperature above the sintering temperature of the refractorymetal. In practice, substantial insolubility in the present processoccurs when the inert metal is less than about 1% soluble in therefractory metal. The inert metal may be selected from the classconsisting of Groups IB, IIB, IIIA, and IV-A metals. Group IB metalsinclude copper, silver, and gold. Group II-B metals include zinc,cadmium, and mercury, but mercury is effectively excluded because of itslow melting point. Group IIIA metals include aluminum, gallium, indium,and thallium. Group IVA metals include germanium, tin, and lead. Forcompacts pressed from tungsten, molybdenum, or tantalum powder, thepreferred inert metal is copper. The weight concentration of the inertmetal powder in the mixture is in the range from about 5% to about Theparticle size of the fine inert metal also, of course, depends upon theselected pore size and concentration. In the case of the tungstenionizer for the ion engines described above, the fine copper powderhaving a particle size of 400 mesh is used.

The next step of the method involves heating the formed porous body to atemperature above the sintering temperature of the pressed compact andbelow the melting temperature of the inert additive for a time periodadapted to establish a sintering pattern in the formed body. Suchtemperature and time period depend upon the particular refractory metaland inert metal chosen. However, in the case of the ion engine porousplug described above using a mixture of tungsten and copper, thetemperature range is from about 1,000 C. to 1,100 C. and the time periodis about two hours. Such heating should take place in an inert orreducing atmosphere. Thus, in the case of the ion engine tungstenionizer, the heating takes place in a hydrogen atmosphere.

Finally, the formed and presintered body is heated above the meltingtemperature of the inert metal for a time period adapted tosubstantially completely evaporate the inert metal powder and sinter therefractory metal powder to a selected pore size. Thus, such final stepssubstantially completely evaporate the inert metal powder and sinter therefractory metal powder to the selected pore size. Again, thetemperature and time period depend on the refractory metal and inertmetal selected. In the case of the tungsten ionizer, using a mixture oftungsten and copper, a temperature of about 1500 C. is used to evaporatethe copper while a temperature of about 1800 C. is used to sinter thetungsten. The time period for the copper evaporation is about one halfhour while the time period for the tungsten sintering is about one-halfto one hour. The evaporation of the copper should be conducted in avacuum such as under a pressure of about 10 torr until the copper iscompletely removed. The sintering of the tungsten should be conducted inan inert or reducing atmosphere or in a vacuum and in the case of thetungsten ionizers, a vacuum is used. It should be noted that the finalheating step is conveniently considered to be a single heating stepalthough the initial portion of it is preferably conducted at atemperature lower than the final portion of it; however, the heating maybe conducted as a single continuous step. Similarly, the presinteringheating step and the evaporation and sintering heating step may beconsidered a single heating step and be conducted as one continuousheating step with the temperature being initially relatively low andsuccessively increased during the subsequent portions of the heatingstep. However, it has been found convenient to conduct the pre-sinteringheating step in an inert or reducing atmosphere furnace and thenconducting the evaporation and sintering heating step in a vacuumfurnace with an intermediate cooling step to permit transfertherebetween.

The porous body produced by the foregoing method is a sinteredrefractory metal powder having a high porosity and permeability. Becauseof the shape and distribution of the inert metal powder, substantiallyall of the pores are elongated so that the length to width ratio isgreater than about 5. The ratio of the surface pore area to surfacesolid area is about 0.60 with an initial weight concentration of copperin the tungsten and copper mixture of about 8%. Also, the pore networkin the porous body has substantially improved continuity and geometrywhich facilitates the vapor flow therethrough and electron transfer.

Another method of the present invention for producing a stable porousbody having a selected pore size and concentration and large surfacearea includes forming a porous body comprising a mixture of finerefractory metal powder and fine boron composition powder. Therefractory metal may be selected as described in the foregoing methodand the particle size may be in the same range. The boron compositionwhich may be used in such mixture may be elemental boron or a boroncompound such as boron nitride or boron carbide or a mixture thereof.The preferred boron composition is boron nitride because of itsresistance to oxidation and the ease with which it forms fine powders.The weight concentration of boron in said mixture is in the range offrom about 0.5% to about 3%. Thus, in the case of a boron compound suchas boron nitride, the weight concentration of the boron composition mustbe correspondingly increased to insure that the concentration of theboron is sufficient. With respect to particle size, it has been foundwhen using boron nitride that a particle size of 325 mesh is adequatelysmall. After forming said body, the next step of the method is to heatsuch body to a temperature above the sintering temperature of therefractory metal for a time period adapted to sinter the refractorymetal powder to a selected pore size. In the case of the tungstenionizer described above, a temperature of about 1800 C. for a timeperiod of about 30 minutes was found to be sufiicient. The porous bodyproduced by such method has a stable high porosity and permeability andcomprises a sintered refractory metal powder having substantially all ofits surface area coated by a boron composition. In

the case of the tungsten ionizer using a boron nitride composition, amajor portion of the coating appears to be primarily a tungsten boridecompound while the minor portion is a complex compound of tungsten,boron, and nitrogen.

Still another method of the present invention for producing a stableporous body having a selected pore size and concentration and largesurface area comprises forming a porous body comprising a mixture offine refractory metal powder, fine boron composition powder, and fineinert metal powder. Each of the constituents of such mixture is the sameas described in the foregoing methods and is present in the same weightconcentrations. The formed porous body is then heated in the mannerdescribed in the first method set forth above. The porous body resultingfrom such method comprises a sintered refractory metal powder withsubstantially all of its pores being elongated so that the length towidth ratio is greater than about five and with substantially all of itssurface coated by a boron composition.

Some specific examples of the method of the present invention and theresulting porous body are as follows:

EXAMPLE 1 A mixture was formed of angular tungsten powder having aparticle size of about 0.8 micron and copper powder having a particlesize of 400 mesh with the weight concentration of the copper in suchmixture being 8%. The mixture was placed in a rubber bag andhydrostatically pressed at 60,000 p.s.i. After pressing, the compact waspresintered in hydrogen for two hours at 1050 C. After cooling inhydrogen, the compact was placed in a vacuum furnace and the copperevaporated by heating to 1500" C. under a pressure of torr until thecopper was completely removed. The temperature was then raised to 1800C. for 30 minutes to complete the sintering. The resulting body was thenused as an ionizer in an ion engine utilizing cesium vapor. It was foundto have very uniform permeability (tested with nitrogen) with a densityin the range of 64-67% of theoretical density. The average pore size wasabout 3.9 microns and the pore density was only about 1.7 million poresper square centimeter. When used in the ion engine, an ion current ofabout 20-28 ma./cm. was obtained. The neutral fraction was 0.1% or less,which was 5 to 10 times lower than the best results obtained with porousbodies formed from fine spherical powder tungsten.

EXAMPLE 2 A mixture of fine angular tungsten powder having a particlesize of about 0.8 micron, fine copper powder having a particle size of400 mesh, and fine boron nitride powder having a particle size of 325mesh was formed with the weight concentration to copper being 8% andboron nitride being 4% (2% boron). The mixture was processed as setforth in Example 1 and the resulting porous body was tested as anionizer in an ion engine using cesium vapor. The resulting porous bodywas found to have a very low density of 48% of the theoretical densityand a permeability which was substantially unchanged when heated at 1600C. in a vacuum of 10* torr for 8 hours while a comparable sphericalpowder tungsten body had a permanent permeability reduction of 35%during such time period with the permeability continuing to drop.

All of the methods of the present invention and the products producedthereby have been described particularly with respect to ionizers forion engines; however, such methods and products have many other obviousfields of use. One immediate application of such methods and products isfor high density electron sources in high power tube devices such astraveling wave tubes. Other applications of the methods and products ofthe present invention include refractory structures such as lampfilaments, rocket nozzles, high temperature bearings, and turbineblades. In many such structures, it is necessary to cool them byprocesses such as transpiration cooling wherein liquid flows through theporous structure and evaporates to protect the surface of the structure.Under such circumstance, stable, uniform permeability during hightemperature operation is essential, and the method and products of thepresent invention achieves such result.

There are many features in the present invention which clearly show thesignificant advance the present invention represents over the prior art.Consequently, only a few of the more outstanding features will bepointed out to illustrate the unexpected and unusual results attained bythe present invention. One feature of the present invention is the useof an inert high melting metal powder in a mixture with a refractorymetal powder in the process of forming a sintered porous body so that apre-sintering pattern is established in the body prior to the finalsintering step. By such technique, ordinary, relatively inexpensive,refractory metal powder having a high surface free energy may beutilized to form a sintered body without the usual rapid densificationof such body at high operating temperatures. Also, the resulting porousbody has uniform elongated pores and a high pore area on its surface sothat exceptionally uniform permeability is obtained. Likewise, when usedas an ionizer, exceptionally uniform ion emission from the surface isalso obtained. Another feature of the present invention is the use of aboron composition powder in a mixture with a refractory metal powder toproduce a stable porous body having a low density. Thus, where apreformed pore is not needed or desired, the inert metal may be omittedand only boron or boron nitride is used in a mixture with the refractorymetal powder (e.g., tungsten, molybdenum, tantalum) so as to produce asintered porous body of high thermal stability. Such body maintains itspermeability for high operating temperatures over long time periodsunlike previous porous bodies whose permeability is reduced verysubstantially. Still another feature of the present invention is thecombination of an inert, high melting metal powder and a boroncomposition powder with a mixture with a refractory metal powder to forma porous body. Such body has the combined desirable characteristicsdescribed above so that it represents a very substantial advance overthe available methods and products and yet may be achieved relativelyinexpensively.

It will be understood that the foregoing description and examples areonly illustrative of the present invention and it is not intended thatthe invention be limited thereto. All substitutions, alterations, andmodifications of the present invention which come within the scope ofthe following claims or to which the present invention is readilysusceptible without departing from the spirit and scope of thisdisclosure are considered part of the present invention.

What is claimed is:

1. A method for producing a porous body having a selected pore size andconcentration and large surface area comprising:

(a) forming a porous body comprising a mixture of fine refractory metalpowder and a material selected from the group consisting of (i) fineinert metal powder and (ii) a mixture of fine inert metal powder andfine boron composition powder, said inert metal being substantiallyinsoluble in said refractory metal and having a melting temperatureabove the sintering temperature of said refractory metals;

(b) heating said body to a temperature above said sintering temperatureand below said melting temperature for a time period adapted toestablish a sintering pattern in said body; and

(c) heating said body to a temperature above said melting temperaturefor a time period adapted to substantially completely evaporate saidinert metal powder and sinter said refractory metal powder to a selectedpore size.

2. The method as stated in claim 1 wherein the weight concentration ofsaid inert metal powder in said initial porous body mixture is in therange of about 5% to about 15%.

3. The method as stated in claim 1 wherein the heating step ofsubparagraph (0) comprises heating said body at a first temperatureabove said melting point for a first period of time adapted tosubstantially completely evaporate said inert metal powder and thenheating said body at a second temperature above said first temperaturefor a second period of time to complete the sintering of said refractorymetal powder to said selected pore size.

4. The method as stated in claim 1 wherein said boron composition isboron nitride.

5. The method as stated in claim 1 wherein said inert metal powder iscopper.

6. A method for producing a porous body having a selected pore size andconcentration and large surface area comprising:

(a) forming a porous body comprising a mixture of fine refractory metalpowder and fine inert metal powder, said inert metal being substantiallyinsoluble in said refractory metal and having a melting temperatureabove the sintering temperature of said refractory metal;

(b) heating said body to a temperature above said sintering temperatureand below said melting temperature for a time period adapted toestablish a sintering pattern in said body; and

(c) heating said .body to a temperature above said melting temperaturefor a time period adapted to substantially completely evaporate saidinert metal powder and sinter said refractory metal powder to a selectedpore size whereby said inert metal powder is substantially completelyevaporated and said refractory metal powder is sintered to said selectedpore size.

7. A method as stated in claim 6 wherein said refractory metal isselected from the class consisting of Groups IV-B, VB, VI-B, VII-B, andVIII metals.

8. A method as stated in claim 7 wherein said refractory metal istungsten.

9. A method as stated in claim 7 wherein said refractory metal ismolybdenum.

10. A method as stated in claim 6 wherein said inert metal is selectedfrom the class consisting of Groups IB, II-B, IIIA, and IVA metals.

11. A method as stated in claim 10 wherein said inert metal is copper.

12. A method as stated in claim 6 wherein the weight concentration ofsaid inert metal powder in said mixture is in the range of about 5% toabout 13. A method for producing a stable porous body having a selectedpore size and concentration and large surface area comprising:

(a) forming a porous body comprising a mixture of fine refractory metalpowder, fine boron composition powder, and fine inert metal powder, saidinert metal being substantially insoluble in said refractory metal andhaving a melting temperature above the sintering temperature of saidrefractory metal;

(b) heating said body to a temperature above said sintering temperatureand below said melting temperature for a time period adapted toestablish a sintering pattern in said body; and

(c) heating said body to a temperature above said melting temperaturefor a time period adapted to substantially evaporate completely saidinert metal powder and sinter said refractory metal powder to a selectedpore size whereby said inert metal powder is substantially completelyevaporated and said refractory metal powder is sintered to said selectedpore size.

14. A method as stated in claim 13 wherein said refractory metal isselected from the class consisting of Groups IVB, VB, VIB, VII-B, andVIII metals.

15. A method as stated in claim 14 wherein said refractory metal istungsten.

16. A method as stated in claim 14 wherein said refractory metal ismolybdenum.

17. A method as stated in claim 13 wherein said inert metal is selectedfrom the class consisting of Groups I-B, IIB, IIIA, and IV-A metals.

18. A method as stated in claim 17 wherein said inert metal is copper.

19. A method as stated in claim 13 wherein the weight concentration ofsaid inert metal powder in said mixture is in the range of from about 5%to about 15%.

. 20. A method as stated in claim 13 wherein the weight concentration ofboron in said mixture is in the range from about 0.5% to about 3 21. Amethod as stated in claim 13 wherein said boron composition is boron.

22. A method as stated in claim 13 wherein said boron composition isboron nitride.

References Cited UNITED STATES PATENTS 2,744,011 5/1956 Samual -222 X3,360,347 7/1964 Todd 75222 X 3,397,968 8/1968 Lavendel 75222 X CARL D.QUARFORTH, Primary Examiner A. I. STEINER, Assistant Examiner US Cl.X.R. 29182.5; 75222

